HTML5 reasons
Deadly Ernest is correct - the appeal of HTML or HTML5 is not that the web technology stack is great, because it's not. It's that the software will run in any browser (if you are judicious in your use of libraries). There are pre-AJAX web apps written in the mid 1990s that could still work today. I wrote a lightly ajaxy web app in 2008 that still runs, but we didn't use it for various reasons. Still, four years is not bad. I still have one script from around 2005 that would be useful given the right context. I'm using a crufty CMS from 2000 to run a website.
We apps have a long useful life. Recent trends to strip out more layout and formatting, and turn the server side into a REST service, will make the apps last even longer.
The web has become, more or less, the achievement of the goal of OO: cross-language remote procedure calls over the network, with totally encapsulated objects that communicate via messages.
